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A project of Volunteers in AsiaSmall Scale Foundries for Developing Countries:A Guide to Process Selectionby J.D. HarperPublished by:

Intermediate Technology Publications9 King StreetLondon WC2E 8HNENGLANDAvailable from:

same as above

Reproduced by permission.Reproduction of this microfiche document in anyform is subject to the same restrictions as thoseof the original document.



E

An Intermediate Technology Publication

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AcknowledgementsThe printing of this publication hasbeen made possible by a grant from theOverseas Developmen Administration.The Intermediate Technology Develop-ment Group gratefuiiy acknowledgestheir generosily.

CC)ntermediate Tecktnnlogy I4iubiicationsLtd, -l-981Written by Geoffrey Lamb (Consultants)Ltd of Birmingham, U.K. for IntermediateTechnology Industrial Services, Rugby,U.K. Published by Intermediate Technol-ogy Publications Ltd, 9 King Street,London WC2E 8HN, U.K.

ISBN 0 9(:1033 8 -Printed by The I wt:: 2:~ s Ltd., Bertrand Russell House, Gamble Street, Nottingham (UK).

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CGNTENTS

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Introduction 1The role of the foundryObject of this bookFoundry processesPattern-making 3Existing castings as patternsPattern designPattern materialsPattern-making eyuipmentPattern storageMoulding 7Choice of moulding methodRunning and feedingMoulding boxesPermanent mouldingCentrifugal castingClay-bonded sand mouldsClay and moistureSand preparationTypes of sandHand mouldinpMachine mouidingMould dryingThe CO, processAir set mouldingOther moulding processesMould assembly and pouringCoremaking 28Core bindersCore makingCore assembly mouldingMelting cast iron 32Technical requirementsRaw materialsCupola furnacesGas and oil furnaces

Chapter 6Chapter 7

Chapter 8Chapter 9Chapter 10Chapter 11

Cnapter i2

Chapter 13

Electric meltingRefractory liningsCapacity of melting furnacesSteel castingsNdiiil-ftXiGi?i metal castingsRaw materialsMelting furnacesMelting aluminiumMelting bronzePouringMoulding and coremakingCleaning castingsInspectionSafetyFoundry planningSpecificationSite and buildingsLayoutSources of foundryequipment and materialsPurpose built equipmentLocaiiy made equipmentImported raw materialsLocal raw materialsSkills and training

Selected Bibliography and ReferencesConversion factorsGlossaryAppendix: Assistance questionnaire

4142

48505152

54

5658606164

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List of illustrations PagesUsing a ladle to pour a row of snap-flaskboxless mouldsStages n moulding a cored castingParts of a typical mould (sectional view)A runner systemMoulding boxesAlt~rnati\~ptwn~cnfmot~ dingh~~. I..-...-..I. - ., y-4 ..- ---Metal die or permanent mouldA simple sand drierSand millSome hand moulders toolsJolt-squeeze moulding machineCarbon dioxide (CO?) process gassingsystemsContinuous mixer-filler for air-set processShell mouldingTwo types of core sand mixerDiagram of cupola furnaceSmall cupola furnace(cupolette)Oil fired rotary furnaceOil-fired crucible furnaceCoke or charcoal fired crucible furnaceCrucible in carrier for useas a ad eDouble-ended grinding machine forcleaning castings

Cove1689111214151713202224253035373843444549



CI-LMTER 1INTRODUCTION

The Role Of The FoundryCastings in iron, brass, aluminium, or othermetals are an essential part of most engi-neering products, and a foundry in which tomake them is needed by any developingindustrial society. Although the productionof castings on a large scale s a sophisticatedand capital intensive business, here can be auseful market for small scale oundries pro-ducing castings for building and domesticproducts, machinery parts, and spare partsfor other equipment.Some foundries are independent organi-sations producing castings for a number ofcustomers. Other foundries are departmentsof larger concerns, which need their ownsource of castings for their products, or forthe production of spare parts.Foundries may specialise in one or moretypes or sizes of casting, or produce a widerange of types of product. Some castings areused for standard items such as pipe fittings,manhole covers and cooking stoves, whilstothers have to be made individually for eachapplication.Many foundries have their own machin-ing workshops to produce finished com-ponents or products, while others only makeraw castings.Foundry technology is changing rapidly,for small foundries as well as for large; cast-ing production. is becoming more of asciencewith modern techniques, and less ofa traditional art. Nevertheless many valu-able skills have to be learned and experienceas well as heoretical knowledge is needed bythe foundryman.Some of the more commonly requiredtypes of casting which might be produced by

small scale foundries are listed below.Such a list can never be complete - one ofthe advantages of the foundry process s itsflexibility, and the possibility of making allsorts and types of casting for a wide range ofapplications.

Cast IronCastingsStovesPulleysManholecoversPipe fittingsPumpsFire barsBrakedrumsVehicleSparesBearingblocksValvesMachineryspares

AluminiumCastingsLevers &handlesFan andmotorhousingsCookingpots andkitchentoolsPortablepumpbodiesPulleysIrrigationpipe fittingsLightfittingsDoorfurniture/Machineryspare partsfor all typesofequipment

Brass &BronzeCastingsValves &taps forcorrosiveliquidsBearingsand bushesBoat partsandpropellorsPumpbodies andpumpimpellorsOrnamentalanddecorativecastingsDoorfurnitureMachineryspare partsfor all typesofequipment



Object Of This BookThis book is not intended as a textbook offoundry practice. The purpose is rather toassist anyone about to start or to expand asmall scalp foundry to consider the variousavailable processes, and to select the mostappropriate for the circumstance:,.

An indication is given of the type of r;:wmaterials arid equipment which wiil beneeded, and the degree of training or skillwhich is likely to be required.Foutldq7 ProcessesFoundry processes comprise the followingelements:1. Making a pattern, which is a model of thecasting to be produced. This pattern is

nsed many times in casting production.2. Making a mould from the pattern.Usually the mould is made of sand and isonly used once; but sometimes per-manent metal moulds are used.3. Melting the metal.

4. Pouring the metal into the mould.5. Removing the solidified castings from themould after cooling.6. Cleaning and finishing the casting.Despite thesecommon elements, there is agreat variety of processesavailable for eachstep. Different processesare suitable for dif-ferent sizes and types of casting, for dif-ferent metals, and for different degrees ofmechanisation depending on the scaleof themarket and the number of each casting to bemade. Some processes require costly ravvmaterials or special purpose machinery,whilst others may require more labour ormore skill than others.Few foundries can make all possible typesof casting although, according to the cir-cumstances, some foundries may be morespecialised than others.Depending upon the range of productsrequired, and upon the raw materials andcapital available, it is possible to select themost appropriate processes or any foundry.



Almost all casting production starts with themanufacture of a pattern. Pattern designanci construction has to be carefully con-sidered n relation to the foundry process, asweli as to the design of the finished casting.Much of the skiii in foundry work is con-tained in the skill with which patterns aredesigned and made. Nevertheless., he basicprin*iples are relatively simple. and intel-liger i craftsmen bkilled in worhing withwood or metal can learn an adequateamount of elementary pattern-making witha minimum of training.

Existing Castings as PatternsIt is possible to use an existing casting as apattern. This technique can sometimes beused when a replacement is needed for adamaged part, particularly when dimen-sional accuracy is not very important. How-ever this method cannot readily be used withcomplicated designs, requiring the use ofinternal cores, as described below. Finishedparts have often been machined, so thatcastings produced from them will not carryenough material thickness for their ownmachining operation.Patterns have to be made larger than thecastings o allow for one or two per cent con-traction of the metal on solidification; aconsequenceof this is that the use of an oldcasting for a pattern will produce a new cast-ing which is smaller than the original.Sometimes these problems can be over-come by building up the casting locally withwood, wax, plastic or other material beforeit is used as a pattern. However, it isgenerally more satisfactory and sometimesquite essential to make a new pattern, espe-

cially if more than one or two castings arerequired.Pattern DesignIn designing a new pattern it is usually neces-sary to work from a scale drawing of thecasting to be produced. The pattern designermust decide upon the mould joii;t plane,and upon which, if any, sections of the cast-ings need to be formed by separate cores(internal cavities and sections which containover-hangs which would otherwise preventthe mou:d from being separated from thepattern).The pattern design\- must calculate thedimensions in order to provide sufficientmachining allowance, and to allow for metalcontraction according to the metal or alloyto be used The pattern must be taperedtowards the mould joint to allow it to leavethe sand cleanly in forming the mould. Thepattern may have to include core prints,which are additional sections to form partsof the mould in which the cores are sub-sequently located. The pattern designermust also design the core boxes which areseparate portions of pattern equipment usedto make the cores.The limitations of the foundry processhave to be considered when designingpatterns. Metal sections must not be toothin, nor must the pattern demand themoulding of unsupported thin pieces ofsand. Sharp corners and re-entrant anglesmust be avoided.The construction of the pattern must berelated to the moulding process to be used,whether by hand or by machine, and accord-ing to the size of the casting being made.Patterns used for hand moulding or for the

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production oi >;~??a11uantities of castingsmay be made of wood, while patterns forproduction of castings by moulding machineare usually macie of n,Ttal or pl? +ic.In order to simplify moulding. by what-l:ver process, patterns for repetition pro-duction are often split and mounted onplates or boards at the mould joint level.Several small patterns may be mounted onthe same plates. ThI; technique requiresaccurate location between tile two halves oftile pattern and the mould, to avoid produc-ing offset cross jointed castings. IMount-ing patterns for hand moulding can saveconsiderable skill and effort on the part ofthe moulder. If moulding machines areused, it is essential to mount the patterns,usually on a steel or iron plate made to fit themoulding machine.The quality of a castil;g can never bebetter than the quality of the paiern fromwhich it is made. Good patterns are essentialfor good casting production.Pattern MaterialsMany patterns are made of wood. If onlyone or two castings are required, soft woodmay be used. However if the pattern is to beused o make many castings, or to be storedfor future requirements, the wood must becarefully selected. Close grained wood isneeded, and the wood should be well sea-soned to minimise the risk of warpage ordistortion.Wooden patterns must be strongly con-structed, and be smooth and well finished.Unpainted wood may be used for somemoulding processes, although special lac-quer paints are available for patterns. Somekinds of paint stick to foundry sands and arenot suitable.Metal patterns for repetition castings maybe made of aluminium, from bronze or fromcast iron. Often they are themselves pro-duced as castings from wooden patterns,subsequently hand finished or machinedand polished to produce the workingpattern. Sometimes duplicate patterns canmost conveniently be made from epoxy

resin plastic with fibreglass reinforcement.Using plaster moulds, high quality plasticpattsrns may be produced more easily (andwith less skill) than by many other methods.Good quality wooden patterns, used forhand moulding and stored with care, shouldlast for the production of 50 to 100 mouldswithout major repair. Metal patterns usedfor machine moulding should last formany thousands of moulds. Alumicium ischeaper, but bronze lasts longer and issimpler to modify and repair by soldering.Sometimes, when only one casting isrequired, a pattern is made from foamedpolystyrene (as used for packing). Thismaterial can be cut with a sharp knife, a sawor a heated wire, and joined with glue orpins. Foamed polystyrene patterns are left ixthe mould and not removed. The moitenmetal when poured burns away the poly-styrene (which is largely air) as it enttrs themould cavity. The resulting castings areoften rough but are suitable for someapplications.Pattern-Making EquipmentIn order to make accurate patterns capableof producing high quality castings, it isnecessary that adequate facilities are avail-able. Equipment for making simple engi-neering drawings and the skills to prepare,read and interpret drawings are required.Pattern-making tools include ail con-ventional woodworking and metal workinghand tools. Accurate measuring equipmentis essential (rules, verniers, scalesetc.) and aflat reference surface is necessary.If power tools are available a disc sander,a planing machine, and a band saw willincrease the range of patterns which can bemade, reduce the labour content, andimprove the accuracy of the work. A lathe isneeded for many types of pattern.For metal pattern-making a multi-speeddrilling machine, an accurate lathe, and forsome types of pattern a turret type millingmachine are likely to be most useful.The pattern shop must be well lit, andprovided with adequate benches, vices, and

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tool and material storage space.Parts of patterns must be ocated togetheraccurately. It is also necessary to provideaccurate iocation for coreboxes and formoulding hexes. For these purposes accu-rate bushes (sockets) and pins or dowels areneeded. For the most accurate work theseshould be purchased from specialist manu-facturers and made in hardened steel. Lw-ever, for less critical castings, simple metallocation pegs and bushes may be made inpattern shops. The use of wooden locationpegs s not recommended.A good supply of screwsand bolts of dif-ferent sizes is also necessary as is a good

quality adhesive and a hard setting fillermaterial.Pattern StorageIf repeat casting orders are likely, patternsshould be stored with care - especially sincethe pattern is often (depending upon thecommercial arrangements) the property ofthe foundrys customer. Wooden and metalpatterns shou d be kept on racks away fromthe foundry and the risks of accidentalbruising or da.mage, ire hazards or damp.In designing a foundry, adequate spaceshould be provided for pattern storage.

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Top half mounted pattern

Bottom hplf mounted pattern

Assembling the mauld

Casting ready for cleaning

Stages n Moulding a Cored Casthg6



Choice of Moulding MethodThe method of moulding to be used must berelated to the type of castings o be producedand to the skills and equipment available inthe foundry.Small castings are usually produced insand tnoulds, by hand if the quantities arenot large, or on moulding machines forrepetition work. Larger castings may also bemade with moulding machines, althoughlarge machines are expensive. It will benecessary to handle large moulds with acrane, whether these are made by machineor by hand.Running and feedingThe choice of the moulding method mustalso be related to the methods used to intro-duce the molten metal into the casting cavitythrough the runner system.

A typical runner system consis;? .3f abasin, formed in the top of the mouid, toreceive the metal as it is poured from theladle. From this basin a vertical channel,called a downsprue, leads o the mould jointlevel, wnere horizontal channels known asrunner bars lead to the casting cavity. Themetal flows into the casting cavity from therunner bars through entry positions knownas ingates.As the metal in the casting cools andsolidifies, it contracts. Unless more liquidmetal is able to flow in to keep the cavityfull, the casting will solidify with emptyspaces or porosity. The additional liquidmetal is prcvided by the use of feeders.Feeders are masses of metal, larger in sec-tion than the casting, joined to the ingate,and calculated to remain liquid until afterthe casting is completely solid.

Much of the skill of casting productionlies in the way in which the runner and feedersystems are designed. The metal must flowfreely hito the thinnest sections of the cast-ing, without scouring and washing away thesand. Slag and dirt should be preventedfrom enter:lg the casting. Porosity andshrir,h.lge must be avoided, by carp jdesign of the solidification process, I ;-feeders (and in some caseschins, whit:metal inserts i- ;he mould to acceleratecool-ing lticaliy). P,,Ihe same time the vie 1 of theweight of casn,+ to the weight of runnersystem h. s to be kept as high as po&ole tcminimise meltir :; c,)sts.Different metals, different casting de-signs, different mo ltding methods, anddifferent pattern-m;,.\.ng methods requiredifferent ::rpes c unner system design.Methods correct i ,)I one foundry may not besuitable for another foundry.There are no fixed rules, the best methodsbeing developed by experience and trial anderror.Some foundries rely upon moulders todesign the runner system, whilst in otherfoundries this is the responsibility of thepattern designer or the manager.Even in the small scale foundry, muchtime, effort and cost can be saved f experi-enced advice can be obtained in the field ofrunner system design.Moulding BoxesTo produce most types of mould, preparedmoulding sand is rammed around thepattern. Usually the pattern is set n a frameor moulding box. Moulding boxes may bemade of iron, steel or wood. Mouldingboxes must be accurately constructed, par-

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Tophalfmould

Bottomhalfmould

Poulingbasin

DownmueI I

I

Weight

Parts of a Typical Mould (sectional view)

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. :.::

Runner bars

A Rwmer System

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titularly in regard to the location of the twohalves together. The production of largecastings may require moulding boxes withreinforcement.Reinforcing bars for moulding boxes mayhave to be we ded or clut nto different con-figurations for the production of differentcastings. Moulding box location should beensured with accurate dowels and bushes,aithough for some lesscriticai types of cast-ing location strips and pegs can sometimesbe accurate enough.Some foundries do not use bottom halfmouldir. boxes, embedding the lower halfof the pattern directly into a sand floor.Except for very large castings, this method isnot recommended since accuracy is difficultto maintain, and since it is not easy to ramsand round a lower half pattern. Very simpleflat. castings may be produced withoutmoulding boxes at all, by printing thep.attern nto a prepared sand floor, and then,..-I,,.p~iiiig metal into the open cavity withoutthe use of a top half mould. Not many cast-ings are suitable for production by thisancient process.Mouiding box sizes have to be selected osuit the size of the casting to be made. Oneor more castings may be made in each mould- provided that they are to be cast in thesame metal and poured together.Small castings should be made in mould-ing boxes large enough to give at least 3 cmclearance around the edge of the pattern,and larger castings need more clearance.Small moulds can sometimes be made inspecial moulding boxes which are hinged toopen so that they can be removed from themould after closing. If the sand is strongenough, and the weight of metal to bepoured is not too great, the mouid can standon its own without support until the metal ispoured and the casting is solid.The advantage of this method is that Orliyone moulding box is needed to produce aseries of moulds. Moulding boxes of thistype (snap flasks, or pop-off flasks)may be purchased, or made - if help isavailabie for the design - in a pattern shop.

Permanent MouldingAlthough most castings are made in sandmoulds, which are only used once, it is pos-sibie to use permanent mou ds made ofmetal for the production of a seriesof cast-ings. Th, U b*Y of these permanent moulds ordies is more common for aluminium, zincalloy, lead and brass castings than for othermetals.The metal mould, which may be con-structed from steel or from high quality castiron, must be produced accurately. A wellequipped toolroom or machine shop isneeded. The die should be designed byan experienced tool-or pattern-maker,ensuring that there is adequate taper, properlocation, sufficient thickness of metal towithstand the heat of pouring, correctgating and feeding arrangements, methodsfor locating any cores, and arrangements forclamping the mould together. Permanentmouids may be coated with clay-silicate orgraphite mixtures before use, and are pre-heated.Many die-cast, permanently mouldedparts, are produced on special purposemachines which inject the metal into themould under high pressure. Pressure die-casting tooling and pressure die-castingmachines are specialized and very expensive.The process s suitable for the mass produc-tion at low unit cost of very large numbers ofcomponents in zinc or aluminium.For smaller scale production the gravitydie-casting process may be used. Themoulds are arranged to open and Cioseandbe clamped together manually or mechan-ically, as they are likely to be too hot andheavy for hand operation. Metal is pouredmanually and after solidification the castingic rpmoved..x.

Gravity die-casting of aluminium, brass,lead or cast ron is a process which is particu-larly suitable for the production of simplecasting shapes n large quantities. Althoughthe cost of the metal moulds is high (higherthan the cost of patterns for sand moulding)the fact that no sand, sand preparation, orsand-handling equipment is needed may10



Pair of small mouldmg boxes with handlespins and bushes

................................

Half of a large moulding box with reinforcementbars, location bushes, and trunnions for liftingwith a craneMoulding Boxes



Corner location system

Snap - flask opened for rcmzgaa

A ltemative Types of Moulding Box12



make the process more economical in somecircumstances.Cenir@igaC CastingIf molten metal is poured into a rapidly spin-ning tubular metal permanent mould acylindri::$ casting can be made.On a large scale, this process is used forcast iron and SC (spheroidal graphite)iron water-pipes; smaller centrifugally castbronze tubes are used for making bearings,and cast iron cylinder liners for engines arealso produced by this process.The speed of rotation has to be related tothe casiiligs being made. For generai casr-ings, such as bronze bushes, about 200 rpmis norma , whilst for the highest auaiiry castiron cylinder liners speedsup to 4i)O pm areused.A centrifugal casting machine consists ofa cast ron tubular mould, a motor and driv-ing system for the spif,ning action, remov-able end plates and - usually - a waterspray system to cool the outside of themould. Machines may be purchased in arange of sizes; simple centrifugal castingmachines can be made in a well equippedengineering workshop.This process should not be confused withcontinuous casting in which metai fiowsslowly out of a furnace through a water-cooled collar, emerging as a rod or bar ofsolid metal. Continuous casting is not easyto control and the large amounts of materialproduced mean that it is a process unlikelyto be of interest to smaii scaie foundryenterprises.Clay-Bonded Sand MouldsMost castings are made in sand moulds. Notevery sand is suitable for the production ofcastings. In order to produce a mould, thesand grains must be stuck or bondedtogether. The bond is usually provided byclay. IMar sand deposits contain sufficientnatural. c ay for this bond. Such naturalsands with about 12% to 15% of clay arefound in many-parts of the world. It is pos-sible by laboratory testing to determine

whether a sand bill be suitable for a foundryprocess, although it is usually better to carryout actual trials in a foundry.Many foundries use sand which containsno natural Ciay, and add clay separateiy(usuaily between 5% and iGVo by weightj.This type of sand must be clean, and in par-ticular free from mica, volcanic ash, crushedsea shell or coral. The grain size should befairly uniform, the fineness determining thesmoothness of the casting. The grain shapeshould be round, or sub-angular; windblown sands or sands which have beenwashed as a by-product of mineral extrac-tion are often suitable. Lake sands and riversands are frequently used, but sea beachsand is sometimes contaminated with shellor salt. Sand from some deposits mayrequire washing and cleaning before use, orat very least sieving to remove lumps orforeign matter. Washed sand s likely to con-tain too much water for foundry use andmay require drying - a simple sand dryer isnot difficult to construct.C ay and MoistureWhere separateclay additions are made, thebest type of clay to use is bentonite. Thismaterial is available commercially forfoundry, and also for oil-well use. Fireclayand other types of clay are also used incertain cases.Sand bonded with clay must have theright amount of moisture in order to makegood moulds. The water content dependsupon the amount and type of clay present,varying from about 3% to about 7% byweight. Tile water content can be measuredby accurately weighing a sample of sandbefore and after drying in an oven - otherchemical and electrical methods can ais0 beused in laboratories.Mould DryingMoulds made in clay-bonded sand, with anatural or synthetic clay, are suitable for theproduction of relatively thin sectioned iron,steel and non-ferrous castings without dry-ing. Moulds for heavy castings produced in



Dowel and bushA locationcavity:o metal

Metal Die or Permanent Mould



Dry sandflows OVEcone andthroughgap

loaded into drum

:r

heat from below

A Simple Sand Drier

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clay-bonded sand should be dried beforeuse. Drying may be carried out in ovens, orwith portable mould dryers fired by gas, oilor other fuel. Dried moulds are frequentlycoated with clay-graphite paints and re-dried before use.Clay-bonded sand moulds used withoutdrying are known as green sand moulds.An intermediate process is suitable for:;omecastings, for which a green sand mouldis skin dried by the application of heat fromthe open mould face. A dry sand layer of afew centimetres thickness is formed. Skindried moulds should be poured within ashort time of drying, since moisture fromthe undried green sand can soak through tothe mould face once more.Sand PreparationThe mixing of sand for foundry use s mostimportant. It is possible to mix sand, espe-cially sand bonded with natural clay, byextensive work by treading and with shovelsand sieves. However, for the most efficientresults, sand should be mixed in a roller millor some other type of high intensity mixer.Such mixing and milling is essential for sandwith added synthetic clay.In addition to the sand and the clay, coaldust is often added for iron castings. Up toabout 10% of this material is needed to pro-duce the best casting surface finish. The typeof coal should, if possible, have low ash andsulphur contents. Other carbonaceousmaterials such as powdered pitch and cer-tain oil products may be used nstead of coaldust. It is not usual to add coal dust for steeland certain types of non-ferrous castings.Foundry sand can be used repeatedly.Used sand has to be sieved o remove lumpsof cores and pieces of metal. The sand isthen cooled and mixed with more water,new sand (about 10% of the weight of metalwhich has been poured) and more clay andcoal dust as necessary. To economise onmixing and on additions, such preparedsand can be used as a facing sand for a layer2 cm to 5 cm thick in contact with a patternto form the moald face. Old sand which has

t :en less carefully prepared is used as abacking sand.In preparing a mixture of sand for mould-ing the proportions of new sand, additionalclay if any is required, coal dust andmoisture must be carefully and accuratelyweighed or measured.It is possible to obtain laboratory equip-ment to test and determine sand properties.For simple foundries this equipment maynot be justified, but careful control of mix-ing, of composition and of water contentshould always be exercised.Types Of SandMost foundries use silica sand; however forhigh quality steel castings and for somespecial cores, sand based on heat resistingminerals may be used, despite their highercost in countries which have to import them.Zircon, Chromite and Olivine sands haveparticularly valuable foundry properties,and should be considered for use when theyare available locally.Hand MouldingThe compaction of the sand around thepattern is most simply performed by ram-ming with a wooden hand tool, taking carenot to damage the pattern. As an alternativeto hand ramming for large moulds muchlabour can be saved by using pneumaticrammers if compressed air is available.The sand is compacted uniformly andhard, to give as rigid a mould as possible.Moulds should be permeable to the gaswhich is formed on pouring; if the sand isfine grained it is necessary o vent the mouldusing a wire pricker to make gas escapepassages.If the pattern is not mounted on boards,the hand moulder has to create the mouldjoint bv cutting the sand, ram both halves,and strip the pattern from the mould. Hemust lay any cores, clean the mould by blow-ing with compressed air or bellows and closethe mould together ready for pouring.Skilled hand moulders can make castingsof great complexity from relatively simple



/m-u 1--\i---N 1-/---ill I I\\ I C 1 Plar viewThe rollers work the sand against the base of the mill,whilst the plough scrapes he sand from the walls and the base.

Sand Mill

17



Sand shovel

loose sand from moulds

for cutting

Ladle and carrier

Sieve for facing sand

sand

Wooden hand rammer

Some hand moulders tools19



patterns, using three or more section mouldsto avoid the need for cores, or sometimesusing templates or sectional patterns tomake uniform mould shapesby hand.The making of complex castings by handmoulding requires two or more years oftraining; however simple hand mouldedcastings can be produced after a few weeksexperience.The hand moulders tools include arammer, shovel, sieve, a fine trowel for cut-ting the sand, vent wires, lengths of tube forcutting pouring holes etc. Dry powder (forexample powdered bone dust) is dusted onpatterns and joint surfaces to stop themoulding sand sticking and not separatingcleanly.

The moulder must also make the runnerchannels in the mould when these are notpart of the pattern equipment. Loosewooden pieces should be used to mould theingates into the casting cavity. The runnerchannels and feeders may be cut with atrowel, or moulded from standard loosepieces kept with the moulders equipment.A cut sand surface wi l not be as smooth as amoulded surface, and even f it is smoothedwith the trowel may entail the risk of pro-ducing sand inclusions in castings.Machine MouldingMechanised casting production needsmoulding machines Moulding machinesmay jolt sand into position or squeeze thesand round the pattern. Both methods areoften combined in one machine.Most simple squeeze machines are notuseful for moulds in which each half is morethan about 150-200 cm deep althoughspecial high pressure hydraulic machineshave been designed for deeper moulds.Usually moulding machines are instaliedin pairs, one to make top half mouids andone to make bottom half moulds. There aretypes of moulding machines which can pro-duce complete moulds on one machine. It isalso possible to use one machine to makebottom half moulds, and then later to usethe samemachine with the other half pattern

to make the top half moulds.Moulding machines usually require asupply of compressed air (e.g. at 6 atmo-spheres), although some types operate withself-contained hydrau ic pumps and requireonly to be connected to an electrical supply.Small moulds can be made on simplemoulding machines, in which the squeezepressure s applied by hand with a long lever,using neither pneumatic nor hydraulicpower assistance.Even the simplest moulding machinesprovide a mechanised means of stripping themould from the pattern and if necessaryofturning the mould over during production.Although special purpose mouldingmachines can be very complex pieces ofequipment, a well equipped workshopshould be able (with some assistanceat thedesign stage) to produce a simple machinefrom structural steel, iron castings, andhydraulic or pneumatic cylinders andvalves.One special type of moulding machine isthe sand slinger. Sand slingers containimpellors with spinning blades to hurl thesand at high speed into the moulds. Sandslingers are expensive machines, used forlarge castings, and not usually suitable forsmall scale foundries.It is possible to purchase automaticmoulding machines which produce veryhigh quality moulds at extremely high rates- several hundred complete moulds perhour. Such a moulding machine on its ownhas no value; it must form part of a completemechanised foundry system, including sandpreparation equipment and mould and sandhandling conveyors. The maintenance andthe capital costs of such equipment are highand are only justified when there is a verylarge and assured market for mass produc-tion of castings.Even simple moulding machines are onlysuitable for repetition production of batchesof 20 to 50 or more moulds at a time. Amoulding machine should be regarded as alabour saving device which is capable of pro-ducing high quality castings with less skill

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~4.itis+able,. : :e;e

Jolting table to whichthe pattern plate is

Jolt - SqueezeMO&ding Machine20

Lifting pinsfor strippingmoulds frompattern



than is required by a hand moulder. Never-theless or any moulding machine the invest-ment is relatively high, and is not usuallyjustified unless there is a sure demand forthe castings, and sufficient sand, metal, andspace to ensure that the machine isthoroughly utilized.It is possible to learn to operate mouldingmachines with a few hours practice, whilstthe production of a mould by hand rammingis a much more skilled operation.The CO, ProcessIn recent years many techniques have beendeveloped to supplement the use of tradi-tional foundry sands. These techniquesinvolve using sand bonded with materialsorher than clay.One such method is the CO, or car-bon dioxide process. Ciean dry sand isthoroughly and mechanically mixed withbetween 3% and 7% of sodium silicate(water-glass). This material is available for anumber of industrial processes.The specialgrades produced for foundry purposes arethe most suitable, but are not always essen-tial. The best ratio of silica to soda is 2 : 1 ata specific gravity of 1.4 to 1.7.The mould is rammed with the mixedsand and then the water-glass is caused toharden by passing CO? (carbon dioxide) gasthrough the mould. CO, gas in cylinders isavailable for a variety of purposes as well asfor foundries.It is necessary o usea reducing valve fromthe cylinder in order to control the pressureof the CO, gas to about 1.5 to 2.0 atmo-spheres.The gas ;. fed into the sand throughthin metal tubes or under a board or plasticcover. Care has to be taken not to waste CO,gas. The gassing time depends upon the sizeof the mouid or core - from half a minuteup to about five minutes. About one or twocubic metres of CO, gas are needed for every100 kg of sand - or 1 kg for every kg ofsodium silicate.Sometimes CO, sand mixtures include asmall amount of clay or other bonding mate-rial so that the mould may be removed from

the pattern without collapse before harden-ing; in other cases he mould is gassedwhilestill in contact with the pattern.Water-glass and carbon dioxide are rela-tively cheap raw materiais. The sand cannotreadily be re-used. Water-glass bondedmoulds may also be hardened by specialchemicals (esters such as glycerol acetates)or by stove drying. However CO1 gas ifavailable is generally more satisfactory. TheCO, process is suitable for all sizes ofcastings and generally requires lessskill thanclay-bonded sand moulding. It may be usedfor cores as weil as for moulds.CO, moulds and cores often produce arough casting finish and for high qualitywork should be painted with carbonaceousmaterial (graphite mixed with clay or otherbonding agents suspended in water oralcohol. The water must be dried or thealcoho burned off before closing themouids). CO> castings often require morework at the casting cleaning stage han greensand castings.For some castings a CO, sand layer of 5cm to 20 cm may be backed with a clay-bonded green sand. Such moulds arecheaper, although less strong and rigid thanfull CO, moulds. They should not be left tostand for longperiods before casting asmoisture can soak through to the CO: layerand so spoil the hard surface.Air-set MouldingA useful method for making a variety oflarge and of small moulds without the use ofskilled iabour or expensive machinery is theuse of chemically bonded air-setting sand.Clean, dry, clay-free sand (with no shell orlimestone contamination) is mixed withabout 1 Vz to 2% of special chemical resinand a hardener, in such proportions thatwithin a few minutes a hard bond is formed.Before the resin hardens the sand flowseasily and needsonly a minimum of packingor ramming around the pattern. Thechemical agents may be mixed with the sandin continuous screw feed mixers; these areusually relatively simple pieces of equip-



. ...... . . . ........ ::::.... .............................................................................................................................................

......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... .............................................. .............................................................................................................................................................................

Gas cylinder withpressure reducingvalveGassing a coreGassing cover fora mould with aprobetube

Ozrbon Dioxide (CO,) Process GassingSystems

22



ment, available from many manufacturers.An important advantage of the air-setprocess s that ihe capital cost for a mould-ing r-,1-A:--iiiSksliclLI~ii, ~ar&u aily for iargecastings, is very much less than the capitalcost of sand-milling plant and mouldingmachines for clay-bonded sands. The skillrequired to operate the process and the skillrequired to maintain the equipment is alsosignificantly less. On the other hand it isnecessary o purchase the special and expen-sive chemicals.It is necessary o choose resins which aresuitable for the metals to be cast. Some typesof resin deteriorate rapidly if they are storedin very warm or in very cold conditions, sothat care must be taken to purchase limitedquantities at a time, and of a type suitablefor the climate.An advantage of the process s that simplewooden patterns can be used. Cores can alsobe produced on the same equipment in thesame sand.If the production of fairly large quantitiesof meuids is necessary, t is possible to setthe mixer over a length of roller conveyoralong which patterns and moulds can bepushed continuously for filling.The economics of the use of chemically-bonded sands depends very largely upon thecost of resins and the ratio of metal pouredto sand used. By careful design of thepatterns and mouids, avoiding the use ofunnecessaryquantities of bonded sand, andby minimizing the waste, the economics ofthe process can be improved.Ch~micG y=bc~ded bard is tis~d byfoundries producing moulds in mouldingboxes and aiso by others which do not usemouiding boxes but assemtile the mouldsfrom blocks or assemble separate cores,according to the design and construction ofthe patterns.Depending upon the type of resin orbinder used, it is possible to crush andreclaim and re-use a proportion of the sand.The economics of reclamation depend uponthe cost of new sand in each area.The binder chemicals normally used are

based on furfuryl alcohol-phenol formaide-hyde resins specially made for foundry use.Other resin systems, and sodium silicate-~. -.(water;giassj hdrberiedwith special catalystsare also used. Mould curing times can bevaried by selection of chemicals, by controlof the temperature, and by varying thehardener or cataiyst addition. Curing timesfrom as ittle as 4 minutes up to as much as 1hour can be obtained.Other Moulding ProcessesThe moulding processes considered abovecomprise those which would normally beconsidered by most foundries. Howevermention can be made of some other mould-ing processes:Investment CastingThe lost wax or investment process is anancient and weli known method of produc-ing castings. A pattern is produced fromwax, often in an accurately made metal die.The wax is coated by dipping (investment) inlayers of Ciay, plaster or special refractory.After the plaster has dried the mould isheated to melt the wax. The wax runs outand molten metal is poured into the result-ing cavity. This process is expensive inmaterials but produces very fine and accu-rate castings. It is often applied for jewelieryand sculpture and when exceptionally accu-rate small engineering castings for jetengines, sewing machines, and other suchproducts are required.Sheii MouiaingShell moulding requires the use of iron orsteel patterns. These are heated to about250C and then covered (either by dumpingor blowing using a special machine) withsand which has been previously mixed with370 to 570of a heat-curing phenolic resin.After a few minutes a hardened layer of 1 cmor 2 cm builds up on the pattern; the loosesand behind is tipped away for re-use andthe hardened shell is removed when cured byfurther heating. Two sheik are clamped orglued together for pouring.

23



Sandhopper

Continuous Mixer-Filler for Airset Process

24



Zattcrr. on .flate

Frame placed overthe heated pattern

Filled with resincoated sand W II&cures against thehot pattern

Excess sand removedleaving curedshell on pattern

Completed half shellmould strippedf;Oiil pattern

~

Shell M&ding

25



The powdered resin may be simply mixedwith the sand but it is more effective to usesand whose grains have been coated withresin. It is possible to purchase coated sand:although the coating process is not verycomplicated, it requires careful technicalcontrol and is not usually worthwhile forsmall quantities.Shell moulding requires relatively littleskili from the operators although it demandshigh skills from the pattern makers.The process can produce accurate,smooth, high quality castings in most typesof metal. Shell moulds can be stored for longperiods without damage and the process canbe mechanised in many ways for high pro-duction.Shell moulding is not a cheap method,especially if resin coated sand has to be pur-chased from a distance. It is suitable onlywhen reiati\d~~ highly priced castings are tobe made which justify the cost of the resinsand the expensive metal patterns.

Plaster MouldsCastings can be made of aluminium andother low meiting point aiioys in mouidsmade of carefully dried plaster. Thesemoulds are not re-usable. Castings pro-duced in this way are accurate although theprOCessSexpensive. S@ai t)p?S Cf PliXSteiare available with which it is possible tomake iron and even steel castings whenexceptional accuracy is required.Cement SandsSand may be bonded with cement. OrdinaryPortland cement may be used, although forthick section castings or those that must bepoured at a high temperature, heat resistingcement (ciment fondu) is necessary. About7% to 9% of cement and 6% to 7% of wateris mixed with the sand. Before pouringcement moulds must be carefully dried.Cement-bonded sand moulds are strong andare suitable for producing heavy castings;however the sand does not break downeasily and this can cause problems withcleaning the castings and with cracking of

thin metal sections. The process is not usedby foundries producing small castings, but isone of the processes o be considered forheavy products.Loam MouldingA variation of the dry sand moulding pro-cess or large castings s the use of loam: thisis sand mixed with a very large amount ofclay such as fire clay, so that it can beapplied by hand in a plastic form against apattern. Loam moulds are thoroughly driedbefore pouring. This is the traditional pro-cess or making heavy castings (pipes, bellsetc.); it demands a high degree of skill andtraining and is not likely to be useful forsmall scale foundries in normal circum-siances.Other BindersWith recent developments in chemistry agreat variety of compounds has been usedfor bonding sand for foundry purposes.Many of these chemicals are expensive andpresent few advantages over conventionalmaterials.One interesting experiment has been touse ce. Sand is mixed with a little water andthe mould is frozen. The metal is pouredbefore the frozen mould has had time to col-larse and good castings can be produced.However the necessaryhigh capacity deep-freezing apparatus is very expensive to buyand to operate.Un-bonded sand may also be held in placeby vacuum suction between special thinplastic films. This (V) process s expensive oengineer and is not usually suitable for smallscale foundries. It is used for repetition pro-duction of large flat castings.Mould Assembly and PouringMoulds must be correctly prepared forpouring, by whatever process they havebeen produced. They should be inspectedfor damage and any loose sand or dirt blownaway with compressed air or bellows. Thecores are laid in the core prints which holdthem in the correct position. The top half of

26



+I..Lllc moi.dC: s closed carefdly and siowiy byhand or with a crane (which must be equip-ped with slow speed controls).Moulds must be clamped securely ogetheror a heavy weight must be laid on top inorder to prevent the molten metal lifting andparting the two halves of the mould. Pour-ing basins n the tops of the moulds must bekept clean and moulds should not be

al owed to get damp before pouring. Pour-ing of molten metal (clean and free of slag,in a clean pre-heated ladle, at the right tem-perature) should be steady and fast enoughto fill the moulds without entraining of airor scum from the metal surface. Consider-able practice is needed in order to pourmetal correctly and safely.



CHAPTER 4

.

COREMAKING

Certain types of casting can be producedwithout the need for cores. However manydesigns contain internal cavities or under-cuts which cannot be made by normalmoulding methods. Very few foundries canoperate without some core producingfacilities.

been developed for producing cores which

The bonding of sand to form cores issimilar to that which is required in order toform moulds; many of the same processesare used. There are, however, some differ-ences. A core has to be harder and strongerthan a mould since it must be handled andperhaps stored, handled again and laid intothe mould by hand. Cores are often almostcompletely surrounded by molten metal andas the metal solidifies and contracts the coremust break down in order not to set upstressesor crack the metal. Cores must notbe bonded with chemicals which producelarge quantities of gas when subjected toI^^^ :-Lheat since gas bubbles can pass IIIL~ theliquid metal and form blow holes onsolidification.Core BindersCores may be made from clay-bonded sand.However clay-bonded cores are very fragileand require considerable skill in man-ufacture and support in handling. The useof green sand and dried satd cores hasdecreasedconsiderably in recc IPyears.The CQ process using water-glass andcarbon dioxide gas is an extremely usefulcoremaking process. The technique isdescribed in Chapter 3. CO, process corescan be made with special sand additives toreduce the casting cracking problem whichsometimes occurs.A number of other chemical systems have

28

harden rapidly in their core boxes withoutheat. A number of chemical resin systemsbased on phenol formaldehyde, urethane,furfuryl alcohol or other resins harden onthe passage of amine gas or of sulphurdioxide. Other resins harden by the action ofcatalysts mixed with the sand just beforecoremaking, as with the air-set mouldingprocess. Many of these chemicals are expen-sive and some present health hazards so thatthey can only be used with special equip-ment. Smaller foundries should usually con-sider only the CO, process or the air-setmixer-filler process for cold setting core-making.The hot box process needsa heated metalcore box: the sand s mixed with resins whichcure on the application of heat. This processproduces solid cores but in other respects tis similar to the shell moulding process-^_.which quay also be used for making cores.After 1 cm to 15% m of sand has cured shellcores may be emptied out so that hollowcores are produced.Both hot box and shell cores have to bemade on special coremaking machines withheaters for metal core boxes. These pro-cesses are convenient and require littleoperator skill, but the equipment and core-box making and tooling costs are high, sothat the processesare usually restricted tofoundries requiring high production rates.Many cores can be made with naturalbinders. Certain vegetable oils, such as lin-seed oil, cottonwood oil and others, whensubjected to heat (temperatures of the orderof 25WC), become hard and strong.Cores are made in simple wooden or metalcore boxes but complicated shapes may



require support on sand or metal formersuntil they are baked.Other natural organic materials are usedalone or in conjunction with hardening oilsand include starch, flour, molasses, or sugarderivatives. Many types of flour when mixedwith water provide a suitable medium forbonding sand to produce cores. A dis-advantage is that starch-bonded cores tendto produce large quantities of gas onpouring, Frequently a mixture of starch andoil is used to give a combination between aworkable sand before curing and a hard andstrong core after heating. It is possible topurchase specially prepared core bindersand oils. Some of these oils are based onmineral oils and on fish oils as well as onnatural vegetable oils. Ir is worth carryingout experiments with locally available oils asthese may provide more economical core-binders than purchased, specially produced,proprietory materials. A typical mixturemight contain 2% oil, 1YZ% starch, and 2%to 3% of water. Such cores should be curedby baking at 250C for approximately 45minutes in an oven.These oil-bonded sand mixtures may alsobe used for moulding when particularlystrong moulds are needed.

The mixing and preparation of coremak-ing sand must be carefully carried out. It isusually unsatisfactory to attempt to mixsand by hand, and a smail sand mixer of thetype used for mouiding sand, or even aconcrete mixer type, should be used. Thecomposition of core sand mixes must becarefully controlled either by weighing or bymeasuring the ingredients. Cores which arecured by heat or by the passageof CO, gasmust be made under controlled conditions.CO2 process sand hardens slowly in the air,so that to avoid waste it should be mixed insmall batches as it is required.Mixers must be cleaned after use to avoidcontaminating other batches.CoremakingCores may be made in metal, wooden orplastic core boxes. These core boxes are part

of the pattern equipment for the castings.The simplest method of making cores s toram the sand into the core box with awooden rammer. Many cores may needreinforcement with wire or nails in order toprovide internal support. Coremaking forcomplicated shapes is a skilled processrequiring several months of training,although simple cores can be made after afew days or weeks of practice.An alternative method of producing coresis to blow the sand into the core boxes. Coreblowing machines can be bought which aresuitable for both hot and coid core makingprocesses. A range of machines is availablefrom simple manually-operated blowers upto fully automatic equipment for the pro-duction of intricate cores on a large scale.Such coremaking machines require com-pressed air, power or gas servicesand main-tenance, and can usually only be justifiedfor repetition castings in conjunction withmechanised mould production.Cores that have been bonded with oil,starch or some resins must be cured beforeuse. Core stoves may be fired by oil, by gas,by coke, wood or other suitable fuels. It isimportant that air should be allows3 tocirculate within the stove since the curingprocess is by oxidation as well as by theapplication of heat. It is necessary for thecore stove to have reasonably accuratetemperature control.Core Assembly MouldingCoremaking methods are sometimes used toproduce complete moulds. The mixer filleror air-set moulding process described aboveis one such example, but most coremakingprocessescan be used for moulding with theadvantages of flexibility, rigid moulds, easystripping, and absenceof the need or mould-ing boxes. Moulds made from assembliesofcores have to be securely clamped and sealedtogether before pouring the metal.Coremaking sands are usually moreexpensive than moulding sands, so core-moulding is only used when there aredefinite technical advantages.

29



6I.-. .1. : ::1- :...,1:. .&&:;.+F4c7:.:.:..;:. :;-;

Two types of Core Sand Mixer30



SUMMARY OF FEATURES OF MAIN SAND BINDER SYSTEMSClay-bonded sands CO, Air-set Oil & Shell Hot box

Green sand -- ProcessDry sand Process Starch Process ProcessBinders -Material Low Low Moderatecosts High Moderate High HighPattern orcorebox costs Low Low Low Low Low High Highcosts ofmachines Low Hand Low Low Moderate Low High HighHigh Machine Hand HandLevel ofcllillc 1 High Hand,ow Machine......,.~ ~~~ _ .~~..~For smallmoulds YesFor largemoulds No___-For small Nocores

HighYesYes

Yes*

LowYesYesYes

Low Moderate Low LowYes Yes* Yes No..- -~___Yes No No No

Yes* Yes Yes YesFor largecores-__Re-useof sand

NoYes

NoYes

YesSome

YesSome

YesSome

No NoNo No

1 Need forheat tocure No Yes No No Yes Yes YesAccuracyand finishqualityFor highproduction

Fair

Yes

Fair_____

No

Fair

Yes

Fair

Fair

Low

Fair

Good

Yes

Good

Yes

* Possible, but not generally used

31



Cast iron is a general term used to covermany different types of material. The mostcommon form of cast ron is grey iron whichis used for most engineering applications.Malleable iron is a different material oftenused for pipe fittings and other castingswhich need toughness and ductility. Thismaterial requires prolonged heat treatmentand a high degree of technical control.SG iron or nodular iron also requiresgood technical control and laboratory facil-ities. SG iron is a strong and ductile materialused or many high duty applications, some-times replacing steel. It is made by addingmagnesium alloys to molten iron of highpurity (made by melting special rawmaterials, or from steel scrap and refiningthe molten metal). Special processes areneeded to add magnesium safely andefficiently.

Most new iron foundry enterprises willproduce grey iron castings only, at leastinitially.Technical RequirementsThe properties of cast ron and the quality ofiron castings depend upon the compositionof metal. Grey iron, although a commonengineering material, is an extremely com-plicated alloy containing carbon, silicon,manganese, sulphur, phosphorus, and otherelements mixed with the iron. The carbonand the silicon content are particularlyimportant. If the carbon and silicon are lowthe iron will tend to be hard, brittle andunmachinable. On the other hand if the car-bon and silicon contents are too high heavysection castings may be weak and soft.The fluidity of molten iron is improved byhigh phosphorus and carbon contents,

although phosphorus has a weakeningeffect upon iron castings. Other impuritiessuch as chromium and other constituents ofalloy steel can contaminate cast iron andrender it hard and brittle. Lead contamina-tion can also weaken cast iron. It isnecessary for the manganese and sulphurcontents of cast iron to be correctlybalanced in relation to the pouring tempera-ture. If there is not enough manganese, cast-ings will be liable to contain slag inclusionsand blow holes.Cast iron of the following composition islikely to be suitable for foundries making arange of general engineering castings.Carbon content 3.4%Silicon content 2.3%Manganese content 0.6%Sulphur content 0.07% maximumPhosphorus content 0.5% maximumFor high strength and thick section engi-neering castings the carbon, silicon, andphosphorous contents should be lower (e.g.3.2%, 2.190, and 0.2% respectively) whilefor thin section castings for stoves etc. thecarbon and phosphorus contents should behigher (e.g. 3.5%, 1.0%). Other composi-tions are required for other types of cast ron- white iron, malleable iron, SG iron etc.It should be realised that the compositionof cast iron may change during the meltingprocess, especially in rotary furnaces andcupolas where a silicon loss and sometimes acarbon loss has to be compensated for.When pig iron of known composition isused as the main raw material and when themelting process s well controlled, problemsdue to metal composition are not likely toarise. However when cast iron is produced

32



from scrap or when castings with differentrequirements have to be produced it is desir-able that the foundry should have someaccess to basic metallurgical knowledge.Not every foundry can afford a trained andqualified metallurgist, nor a laboratory foranalysis, but many foundries can arrange toobtain occasional analyses and advice asnecessary from universities or technicalinstitutions.There is no substitute for practical experi-ence and training. Without the services ofsomeone who has worked in foundriesbefore, a long learning period must be antic-ipated for any new foundry organisation,even if it is planned to make only thesimplest types of casting.The carbon and silicon contents are themost important elements to control. It ispossible to carry out shop floor tests whichcan give a good indication of the carbon andsilicon content without the necessity for ananalytical laboratory.Casting an accurately formed wedgeshaped piece of iron and breaking it toexamine the fracture can tell an experiencedfoundryman much about the compositionof an iron melt. It is also possible to pur-chase nstruments which register the change

in termperature as samples of iron solidify.The changes n cooling rate are related to thecarbon and silicon content. This equipmentis expensive, although cheaper than alaboratory, but it can give accurate carbonand silicon contents which are invaluable toany foundry attempting to produce ironcastings to a close specification.Raw MaterialsThe best raw material for the production ofcast iron is pig iron. Pig iron is made fromiron ore: many different compositions andqualities of pig iron are produced suitablefor different types of castings.Grey iron castings may also be producedby the use of cast iron scrap as a rawmaterial. Most cast iron is produced from amixture of cast iron scrap and pig iron.Castings made from scrap alone will tend to

have low carbon content and be hard anddifficult to machine. Cast iron scrap isusually available from broken machineryand automobile engines. it is important toensure that the cast iron scrap is not con-taminated - particularly with alloy steelsorother metals.Scrap must be broken into pieces smallenough to suit the melting furnace which isto be used.The composition of cast iron has to bevaried (in particular the carbon silicon andphosphorus contents) in relation to thethickness of the casting and the application.High strength cast iron cannot be producedfrom cast iron scrap containing phos-phorus; phosphorus is often present n scrapcastings which have been made for roadfurniture, gutters, gratings, and heatingstoves. On the other hand phosphorusimproves the fluidity of molten iron, so thatthis type of scrap is useful if thin sectionedcastings are to be made.It is also possible in some circumstancesto use steel scrap for the production ofcast iron. Steel scrap requires the additionof carbon and silicon during the meltingprocess. Steel scrap must be carefully selected to ensure the absence of alloys. Evensmall amounts of chromium plate, of toolsteel or stainless steel, can ruin a melt of castiron.Unless special melting furnaces (electricinduction furnaces) or mixing devices areavailable, only small proportions (e.g. 5%)steel scrap should be used in normal greyiron melting.Any necessarysilicon additions are madeas ferro-silicon. Ferro-silicon is producedfor the steel industry. Large quantities ofpower are needed to produce it, so that it ismade in countries. with hydro-electricresources.Synthetic graphite or coke is sold forre-carburising steel scrap in induction fur-naces. However re-carburisation can also beachieved using charcoal. Re-carburisersmust not contain undesirable amounts ofsulphur, nitrogen or moisture.

33



Cupola FurnacesA large number of different types of furnacemay be used for melting cast iron. Of thesethe coke-fired cupola furnace is the mostcommon. This furnace consists of a shaft,lined with firebrick or fire clay, often with asand bottom rammed on to doors whichopen to discharge the residue after a melt.A coke fire is made in the cupola, andalternate layers of metal (pig iron or scrap)and more coke are charged on top of the bedof incandescent burning coke. Air is blownin through apertures (tuyeres) in the side ofthe furnace. Molten iron accumulates and istapped-out through a hole at the bottom.The tapping hole is usually blocked with asand plug, except when a ladle of iron isbeing tapped out.The coke ash and any other dirt is formedinto a iiquid siag by limestone, which ischarged with the coke. The slag is tappedfrom the back of the cupola periodicallyduring a melt.A cupola furnace is relatively inexpensiveto construct. However the performance of acupola depends upon its design, and expertadvice should be sought if a home-madecupola is being planned. It is essential toprovide a fan of sufficient capacity to main-tain combustion at a high level and ensuresufficient metal temperature. Instrumentsto measure the air volume or pressure arevery useful.As an approximate guide a cupola shouldhave an internal diameter (the diameter ofthe steel shell less a lining thickness of atleast 15cm per side) and be equipped with anair biower as specified below:Tons Per Internal Volume of Air atHour Diameter 0.1 kg/cm2

Pressure1 45 cm 23 cu m / min2 60 cm 40 cu m / min4 75 cm 63 cu m / min6 90 cm 90 cu m / minThe four to six tuyeres should be set fromabout 25 cm to 90 cm above the sand base.The stack above the tuyeres should be at

least 3 metres high plus a taller chimneyabove the charging level to remove thefumes from the foundry.It is also necessary o provide a means ofhoisting the charges to the fmnace sill.If the air supply to a cupola is accuratelymeasured and divided equally between tworows of tuyeres, better and more economicalperformance results. The cost of the equip-ment to achieve this is likely to be justifiedby a coke saving in cupolas which melt formore than 4 to 5 hours at a time, at morethan 2 or 3 tonnes per hour.A full size cupola furnace is not likely tobe economical for melts of less than 3 or 4tons. Some foundries using cupolas producemoulds every day which are left on the floor

for several days until sufficient haveaccumulated to justify a melt.Cupoias melt continuousiy, and do notproduce large batches of molten iron at onetime. Depending upon the size of thefurnace, cupolas can be made to melt atfrom as ittle as 1 tonne per hour up to 30 ormore tonnes per hour.Cupola melting is relatively inexpensive,although it is necessary o obtain supplies ofsuitable coke. Large cupolas require cokeproduced especially for the purpose;cupolas of less han about 2 tonnes per hourcan use other types of coke such as that usedfor blast furnaces or even for domestic coke.The sulphur content of the coke should below. If high sulphur coke is used it will benecessary to add extra ferro manganese tothe charge.Hard charcoal has also been used as acupola fuel, alone or with coke, althoughthe results are not very satisfactory even onsmall cupolas.Cupola coke consumption is about 12%to 15% of the weight of the metal, plus theweight of the initial coke bed for each me t.The operation of the cupola, should notbe undertaken without experienceand train-ing. Cupola melting which is not underproper metallurgical control may producepouring temperatures so low that defectivecastings are produced. Iron may even

34



AiI

Charges n , II

. blast .--IT

Alternate chargesof metal and coke

_ ncandescent 7coke bedTuyeres o- _

Diagram of Cupola Furnace35

Slag holeSand bedDrop botl :om doors



solidify inside the cupola.Some small home made cupolas givegood results, but properly designed furnacesare usually more satisfactory. Very smallcupolas (cupolettes) are often made totilt on trunnions to help in tapping and slag-ging. Larger furnaces do not tilt. Theyrequire experienced operators to conductthe melt so as to control the air, metal andslag flow.Gas And Oil FurnacesIn countries where gas and oil are availablerelatively economically these may be usedfor melting. Gas or oil-fired rotary furnacesare amongst the most efficient ways of usingsuch fuels. Unlike cupolas which producemolten metal continuously, rotary furnacesare batch furnaces. Many rotary furnaceshave a limited size of charging aperture sothat large pieces of scrap cannot be used.Rotary furnaces are equipped with heatexchangers to pre-heat the combustion airwith the escaping flue gas, thus saving fuel.These furnaces can be made with capacitiesfrom a half tonne up to twenty-five tonnesor more. In rotary furnaces there is a carbonand silicon loss so that pig iron charges haveto be used with r-iatively little scrap.

Rotary furnaces require large quantitiesof fuel to pre-heat the linings and are there-fore most efficient if used for several meltsconsecutively. Fuel consumption is about130 o 230 itres of fuel oil per tonne melted.Some manufacturers of rotary furnacesproduce burners which can be used to adaptthe furnaces to run on powdered coalinstead of oil or gas.Gas or oil fired crucible furnaces (asdescribed below for non-ferrous metal melt-ing) can also be used for melting cast iron inbatches of up to fi to % of a tonne. Fuelconsumption would be approximately 0.3 to0.6 litres of oil per kilogramme of iron.Electric MeltingElectric furnaces may be used for meltingsteel scrap, cast iron scrap or pig iron.

Electric furnaces are expensive n themselvesand also require capital expenditure forelectrical supply, transformers, capacitors,and contactors. Nevertheless heir flexibilityand adaptability make them the selectedunit for many foundries, even when electri-cal supplies are not available and it is neces-sary to install generators. The economicsof using an electric furnace depend uponthe cost of energy, which can sometimesbe offset by the use of cheaper or morereadily available raw materials such as steelscrap.Electric furnace maintenance requiresskilled electricians and engineers and expen-sive spare parts. The lining materials usedfor electric furnaces can represent a majorcost. It is unlikely that the special purposelining materials required for inductionfurnaces can be obtained from other thanspecialist sources.As an approximate guide to power con-sumption, a continuously utilized efficientelectric furnace will consume between 500and 600 kilowatt hours per tonne of ironmelted. However with intermittent opera-tion power consumption over twice this levelis not uncommon.The power supply must be secure andcontinuous without risk of interruption.Electric furnace specifications and powersupplies should be discussed with electricalsupply authorities.Types of Electric FurnaceCoreless Induction FurnacesInduction furnaces operating at mediumfrequency (200 to 1000 cycles) are flexibleand convenient and with modern electricalcontrol systems are becoming relatively lessexpensive. Lower frequency (50 to 60 cycles)induction furnaces should operate con-tinuously and are best suited when only onetype of iron is to be melted. These mainsfrequency furnaces are very useful forrecarburising steel scrap, and for meltingthin sheet scrap and swarf.

36



Charge through open top

Air blasttrunnion throughsupports

.d

Small Cupola Furnace (Cupolette 1

37



Chimney

Heated air Heatexchanger

Roller supports

Oil Fired Rotary Furnace

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Channel Type Induction FurnacesThese furnaces are cheaper than corelessfurnaces but are usually less suitable formelting than for holding molten metalmelted until required. They must operatecontinuously for months at a time withoutinterruption.Arc FurnacesThese furnaces are suitable for meltingmany different types of scrap. Thejj aremost efficient when used at full capacity.The carbon electrodes used in arc furnacesare consumable and must be purchased,about 5 to 10 kilogrammes per ton of metalmelted are required. Arc furnaces areparticularly useful for large scale steelmelting.Electric Resistance FurnacesThese are not generally suitable for cast ironalthough for lower melting point metals theyare the simplest type of electric furnace.

Whichever type of electric furnace isselected s bound to be a major investment.This expenditure requires the best technicaladvice and can only be justified by fullutilisation of the furnace. Electric furnacesare not suitable for foundries with no accessto technical, electrical and maintenanceservices.Refractory LiningsWhenever metal is to be melted and handledit is necessary to have the correct heat-resistant lining materials for the furnacesand the ladles. Refractory lining materialsused depend upon the type of furnace.Crucible furnaces require no furtherlining. Cupola furnaces and oil or gas firedfurnaces must be lined with refractorybricks and compounds. Fire bricks and ram-ming materials to a wide variety of specifica-tion are made for different types of duty.Silica-alumina fire bricks are normaily usedfor cupolas and for rotary furnaces.In addition to fire bricks it is necessary ohave heat resistant fire clay available for thepatching and repairing of furnaces. Clay by 39

itself tends to crack and to crumble whendried, and for this reason it is usually mixedwith sand or with crushed fire bricks in orderto make refractory patching materials. Fireclay mixtures of this type are often used forladle linings as well as for furnace repairs.Ladle linings must be most carefully driedand pre-heated in order to ensure that nomoisture remains in the lining. If molteniron is poured onto any damp material thereis a danger of boiling, splashing or explosion.Another method of lining ladles is touse sand. Some naturally bonded mould-ing sands are suitable for lining small lad-les. Silica sand bonded with water-glass,either dried or gassedwith CO, can also givesatisfactory results. For larger ladles hold-ing more than 200 to 300 kilogrammes ofiron it is advisable to use stronger ladlelining materials such as fire brick or fireclay mixtures.Electric arc furnace hearths are lined withsimilar materials to those used for ladles andfor rotary furnaces. I-Iowever, high strengthbricks are necessary or arc furnace roofs.Electric coreless induction furnaces areusually lined with special high purity silicamaterial. This silica is carefully graded andbonded with small proportions of boric

acid. Induction furnace linings are neces-sarily thin and have to be most carefullyinstalled and maintained.Capacity of Melting FurnacesThe capacity of foundry melting equipmentmust be calculated to suit the plannedfoundry output, bearing in mind the follow-ing points:I. YieldAllowance has to be made for melting themetal which fills the runner system (which intypical iron foundries may be from 30% to60% of the poured weight) and for somereject castings, as well as metal which isnever poured into moulds. Iron foundriesmaking heavy castings can expect a yield ofgood castings of between 60130o 70510f themetal melted, whilst foundries making lightcastings may have yields below 5070.



2. TimingThe melting furnaces in non-mechanisedfoundries are unlikely to operate con-tinuously. Depending upon the method oforganisation, the melting furnaces may onlywork for h i of each day or even less thandaily. The furnace capacity must be enoughto melt all that is needed n the working time.3. Pouring WeightThe maximum weight of metal to be poured

into any mould should also be considered.The metal must be melted in a reasonablyshort time. Foundries making small num-bers of large castings are likely to have toinstall furnaces with capacity greater thanthat which would be dictated by their aver-age requirements.Such considerations should also influencethe choice between batch furnaces and con-tinuous furnaces such as cupolas.



The prodtiction of steei castings s more dif-ficult than the production of iron castings,due principally to the fact that the meltingpoint of steel is higher than that of cast ironso that higher melting and pouring tempera-tures are necessary.It is not possible to melt steel in cupolas,nor in rotary furnaces. Steel may be meltedin some types of crucible furnace; howevermost steel foundries use electric furnaces.Before electric furnaces lteie available steelfoundries used cupolas to melt cast iron andthen converted the molten cast iron to steelin converters. Such steei-making processesare now rarely used by steel foundries andare generaliy oniy used on a larger scale forbulk steel production.Medium frequency induction furnaces,although expensive, are the most suitablefor steel castings. Arc furnaces may be usedfor large casting production.If special types of alloy steel are beingmade - for example for wear or corrosionresistance - it may be necessary to usespecial basic (Chromite or Magnesite)furnace linings.Many steels can be made from scrap,although it is likely to be necessary o checkthe composition in a laboratory and to addferro alloys or alloying metals if close spe-cifications have to be met. Control of themelting operation and metal composition iseven more important for steel than for cast.non.Steel castings have different shrinkagebehaviour from that of iron castings.Pattern design, runner systems and othertechniques are different. In particular steel

castings require larger feeders to ensure theabsence of shrinkage cavities. The yield ofgood castings to metal melted is thereforeusually lower for steel than for cast iron.One advantage of steel castings over castiron castings s their weldability, so that theymay be repaired if necessary and joined toother components by welding.Steel castings may be made in greensand moulds (without coal dust), shellmoulds, dry sand, CO, sand and other typesof moulds. Some types of chemical bind-ers may have to be specially selected forsteei castings to avoid the risk of surfacedefects.Most steel castings have to be heat treated(normalised or annealed) aft.er casting orafter weld repair. Heat treatment furnacesfired by oil, gas, electricity or solid fuel,should be well insulated to avoid heatwastage. A typical heat treatment is to heatthe castings to 850C and hold them at thattemperature for two hours. The castings arethen cooled slowly in the furnace to annealthem, or withdrawn from the furnace tocool in the air if they are to be normalised.Furnaces capable of providing controlledheat treatment at this temperature have tobe carefully designed and constructed.In general the production of steel castingsrequires more specialised equipment andmore complicated technical processes handoes the production of grey iron, alumi-nium, or bronze castings.An inexperienced small-scale foundryshould not attempt to produce steei castingswithout comprehensive external advice andassistance.

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Raw MaterialsThe easiest way in which to make non-ferrous castings is to purchase ingots ofmetal of the correct composition for thealloys required, from a refiner or metaldealer. However it is often cheaper to usescrap. The dangers of using scrap metal lie inthe fact that there are many different alloysused for different purposes which cannoteasily be distinguished by their appearance.AlurniniumFor example aluminium castings shouldideally be made from different alloys fromthose which are generaliy clsed for rolledaluminium products. To use scrap which isnot scrap casting alloy may produce defectsin all but very simple castings. Mostaluminium castings are made from alloyswith about 5070, 90, or 13% silicon.The selection of raw materials has todepend upon the type of casting being madeand its requirements. There is the need foraccess o metallurgical advice to deal withspecific problems if high quality castings areto be made using scrap.Brass and BronzeThe most commonly produced copper cast-ing alloys are brass and bronze. There aremany varieties of these but the most usualare the following:1. Brass, consisting of 70% copper and 30%zinc, or 60% copper and 40% zinc, is usedfor valves, pipe fittings, decorative pro-ducts etc.2. Phosphor bronze, containing 88%copper, 12% tin and A% phosphorus ismainly used for bearings.3. Gun met l with 88% COpperi 8% tin and

4% zinc is used for bearings, gears,bushes etc.Foundries making different types of metamust always take care to store the returns,ingots, and scrap separately to avoid con-tamination of one grade with another.Non-ferrous metal casting yields to metalmelted are usually lower than for cast iron,figures of 30% to 50% being normal.Meiting FurnacesNon-ferrous metals may be melted in oii -fired furnaces, gas-fired furnaces, electricfurnaces or crucible furnaces.Crucible FurnacesFor small scale production the use ofcrucible furnaces is the most common melt-ing method. Crucibles are pots made of fireclay mixed with coke dust or graphite, verydensely compacted, carefully dried, andfired at a high temperature. Crucible man-ufacture is a specialised and skilled businessand few foundries produce their owncrucibles.Crucibles are placed in furnaces. Themetal to be melted is charged into thecrucible, which may then be covered with alid. The metal melts as the crucible is heated.When the metal has reached the correctpouring temperature the crucible may beremoved from the furnace with specialtongs, and then transferred to a carrier fromwhich the crucible is used as a ladle to pourthe moulds directly.

me 412 small scale foundries for developing countries04 127 jf en 139200

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